Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, personal digital assistants (PDAs), various wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices.
In particular, MEMS transducers are increasingly being used in portable devices with communication capability, e.g. mobile telephones or the like. Such devices will include at least one antenna for transmitting and receiving RF signals. The amount of power transmitted by such devices can be relatively high and is set to increase with changes to the communication standards. This can cause a problem for MEMS transducers, such as microphones, with CMOS circuitry. The transmitted RF signals can be coupled to the CMOS circuitry and, as the CMOS circuitry is inherently non-linear, such signals may be demodulated to the audio band. This may therefore result in audible noise such as the so-called “bumblebee noise”. This problem may be exacerbated when using MEMS microphones with integrated CMOS circuitry as in many devices the position of the antenna happens to be close to the position where the microphone is required.
As a result of this, the transducer and circuitry are often provided in a package which is at least partly shielded to prevent radiated RF from directly coupling to the CMOS circuitry, e.g. radiated RF noise. However it has been appreciated that, in use, conduction of RF noise into the transducer package and to the circuitry of the transducer die can occur via the electrical contacts of the transducer package which connect an output terminal of the transducer to one or more external components. Filter circuits are therefore often also incorporated into the transducer package for filtering this conducted RF noise.
Typically, filter circuits used to filter the RF signals are low pass RC (resistor-capacitor) filter circuits. Such filter circuits may be provided e.g. between an output/input terminal of the transducer circuitry and external contact of the transducer package. MEMS microphones therefore often require passive radio frequency (“RF”) filters at their pins, e.g. between an output/input terminal of the transducer circuitry and external contact of the transducer package. Traditional implementation of these filters is a low pass resistor-capacitor (“RC”) filter, with a resistor R in series with the pin, and a capacitor C that is shunt to ground.
This traditional circuit presents a problem at direct-current (“DC”) and low frequencies. For example, the resistor potentially limits the maximum current that can be provided to the part which affects, e.g., trimming, so the value of the resistor may have to be selected to be sub-optimal for radio frequency (“RF”) filtering or manufacturing. Furthermore, the resistor adds to the output impedance of an analogue microphone, limiting application-specific integrated circuit (“ASIC”) design choices, and increasing the tolerance of the output impedance, which may make it more challenging to comply with customers' specifications. In some arrangements heat generated by the filter circuit—for example by the resistor of a filter circuit—may affect specified requirements of the transducer package such as the power supply rejection (“PSR”).